1.$OMS_HOME/bin/emctl set property -name oracle.sysman.eml.maxInactiveTime -value -1 -sysman_pwd grid_pwd
Oracle Enterprise Manager 11g Release 1 Grid Control
Copyright (c) 1996, 2010 Oracle Corporation. All rights reserved.
Property oracle.sysman.eml.maxInactiveTime for oms EM_BOX:4889_Management_Service has been set to value -1

#oracle.sysman.eml.maxInactiveTime=time_in_minutes
-1 : Unlimited Duration

2. Restart services to apply changed value

[oracle@EM_BOX config]$ $OMS_HOME/bin/emctl stop oms
[oracle@EM_BOX config]$ $OMS_HOME/bin/emctl start oms